1. ( Field of the Invention)
The present invention relates to an electric motor and more particularly to a structure for suppressing generation of electromagnetic vibration and noise caused by electromagnetic exciting force in the radial direction of an inverter motor.
2. (Prior Art)
Many electric motors are used in household electric appliances and various office automation devices and furthermore in recent years, have been loaded in electric cars.
The motors are required for large output power, thus vibration and noise due to electromagnetic exciting force are caused.
The electromagnetic exciting force in the radial direction is generated when, during mutual movement of a rotor and a stator, the electromagnetic path of magnetic field flux generated from the magnetic poles of the rotor is periodically changed whenever the magnetic poles of the rotor cross the openings of the slots formed on the stator and the magnetic flux distribution in the gap is changed. The time harmonic order, space harmonic order, and amplitude of the electromagnetic exciting force in the radial direction depend on the number of poles of the effective pole opening angle of the rotor and the number of slots formed in the stator.
On the other hand, due to the pursuit of a comfortable environment in a house and a car, a request for reduction in vibration and noise is increased and arts for reducing them have been developed.
As one of the arts for reducing vibration and noise, a skew is formed in the rotor of the electric motor. As a means for forming the skew, the following may be cited.
In Patent Document 1, the outer peripheral surface of the shaft is divided equally into several parts in the axial direction in the process of manufacturing the rotor by fixing the piece-shaped permanent magnets to the positions equally distributed on the outer peripheral surface, and the projections are formed on the outer peripheral surface of the shaft by pressing in the respective divided rows, and the permanent magnets are fixed using the projections for positioning the permanent magnets, and the skews are provided by attaching the balance weights to the tops of the projections when necessary, and insurance of positioning of the permanent magnets and skew angle, simplification of the steps, and reduction in the number of parts are realized.
In Patent Document 2, the same poles of a plurality of ring permanent magnets multi-pole-magnetized in the axial direction are installed opposite to each other at a gap of t by forming a positioning slot in the motor shaft as a guide, thus the rotor magnet is structured so as to direct the magnetic flux lines in the radial direction, and the characteristic of the surface flux density is improved remarkably, and miniaturization and high performance of the motor are realized.
In Patent Document 3, the magnet built-in rotor of the rotating machine is composed of a plurality of divided cylindrical bodies having a plurality of built-in permanent magnets so as to form magnetic poles whose polarity differs alternately in the rotational direction and a shaft for supporting the cylindrical bodies in a laminated state. The divided cylindrical bodies are laminated in the axial direction that the respective magnetic poles are shifted in the rotational direction, are fit into the shaft, are joined in the rotational direction, thereby are connected integrally with each other. By doing this, torque ripples are reduced, and the smooth rotation of the rotor is ensured, and a rotating machine can be obtained at a low cost.
In Patent Document 4, in the brushless DC motor composed of the rotor having a mounted permanent magnets and the stator having a plurality of slots, the effective pole opening angle of the rotor is set to a value obtained by adding the angle equivalent to one slot opening to an integer times of the slot pitch of the stator, and the rotor is divided into several parts in the axial direction, and the divided parts of the rotor are structured so as to be shifted around the shaft by the mechanical angle equivalent to one half of the period regarding to the respective cogging torque.
Patent Document 1: Japanese Application Patent Laid-open No. Hei 6-245417
Patent Document 2: Japanese Application Patent Laid-open No. 2000-245117
Patent Document 3: Japanese Application Patent Laid-open No. 2000-308286
Patent Document 4: Japanese Application Patent Laid-open No. 2001-359266